diff --git a/tool/transform_mjit_header.rb b/tool/transform_mjit_header.rb
index 26ca3d99b4..3c2aa4d234 100644
--- a/tool/transform_mjit_header.rb
+++ b/tool/transform_mjit_header.rb
@@ -14,7 +14,7 @@ module MJITHeader
   #   VALUE foo(int bar)
   #   VALUE __attribute__ ((foo)) bar(int baz)
   #   __attribute__ ((foo)) VALUE bar(int baz)
-  FUNC_HEADER_REGEXP = /\A((\s*#{ATTR_REGEXP})|[^\[{(])*\((#{ATTR_REGEXP}|[^()])*\)(\s*#{ATTR_REGEXP})*\s*/
+  FUNC_HEADER_REGEXP = /\A[^\[{(]*(\s*#{ATTR_REGEXP})*[^\[{(]*\((#{ATTR_REGEXP}|[^()])*\)(\s*#{ATTR_REGEXP})*\s*/
   TARGET_NAME_REGEXP = /\A(rb|ruby|vm|insn|attr)_/
 
   # Predefined macros for compilers which are already supported by MJIT.